1. Field of the Invention
The invention provides an improved refrigeration device. In particular, the improved refrigeration device includes one or more thermal regenerator for exchanging thermal energy with a refrigeration gas with at least one of the thermal regenerator disposed distal from the cold end of the device.
2. Description of Related Art
A cryogenic refrigeration device includes a sealed working volume filled with a working refrigeration fluid, e.g. comprising helium gas. Such a device may be used to cool an element to temperatures below 100° K. (degrees Kelvin). An example refrigeration device 10 of the prior art is shown in section view in FIG. 1. The device 10 is a miniaturized refrigeration device includes a gas compression unit 12 and a volume control unit 14. The compression unit 12 and volume control unit 14 are interconnected by a first fluid conduit 16. The sealed working volume of the example device at least includes the internal volumes of the compressor unit 12, the fluid conduit 16 and the volume control unit 14. Miniature cryogenic refrigeration devices are commercially available that are configured with the gas compression unit, the volume control unit, and the fluid conduit all integrally formed in a unitary assembly such as a crankcase. Examples of these devices are disclosed in U.S. Pat. No. 3,742,719 by Lagodmos, in U.S. Pat. Nos. 5,197,295 and 4,514,987 by Pundak et al, in U.S. Pat. No. 6,327,862 by Hanes, and in U.S. Pat. No. 4,858,442 by Stetson.
Other miniature cryogenic refrigeration devices are commercially available that are configured with the gas compression unit separate from the volume control unit, and with the fluid conduit extended between the separated units. Examples of these devices are disclosed in U.S. Pat. Nos. 5,596,875 and 4,024,727 by Berry et al., in U.S. Pat. No. 4,711,650 by Farie et al. and in U.S. Pat. No. 6,397,605 by Pundak.
In FIG. 1, the conventional gas compression unit 12 comprises a compression cylinder bore 18 formed within a surrounding crankcase 20 and a cylindrical compression piston 22 movably disposed within the compression cylinder bore 18 and movable in response to a driving force applied to the compression piston 22 by a drive link 24. A cylinder head 26 attaches to the crankcase 20 to seal a compression end of the cylinder bore 18. A cylindrical compression volume 28 is formed at the compression end of the cylinder bore 18 between the piston 22 and the cylinder head 26. Reciprocal movement of the piston 22 along a longitudinal axis of the cylinder bore 18 cyclically varies the volume of the compression volume 28, and consequentially cyclically varies the volume of the entire working volume. Accordingly, movement of the piston 22 generates a pressure wave that propagates through the working volume. The pressure wave is generated in the compression volume 28 and propagates through the first fluid conduit 16 to the volume control unit 14 and through the volume control unit 14 to a sealed end thereof. The pressure pulse is reflected by the sealed end and propagates back towards the compression volume 28. Accordingly, the refrigeration gas flows bi-directionally through the working volume with peak pressure amplitudes occurring as the piston 22 is driven toward the cylinder head 26 and with minimum pressure amplitudes occurring as the piston 22 is drawn away from the cylinder head 26.
The volume control unit 14 comprises a cylinder housing 30 formed to surround a longitudinal bore or cylinder 32. The cylinder 32 is open at one end to receive a gas displacing piston 36 therein and is sealed at a closed end by an end cap 34. The gas displacing piston 36 is movable within the cylinder 32 and is reciprocally driven along the cylinder longitudinal axis by a drive link 38. Movement of the gas displacing piston 36 cyclically varies the volume of a gas expansion space 40 formed between the inner most end of the gas displacing piston 36 and the end cap 34. Each cycle of the refrigeration device 10 cools refrigeration gas contained within the expansion space 40. An element to be cooled 42 attaches to the end cap 34 and cooled by the refrigeration gas inside the expansion space 40. A fluid port 44 provides fluid communication between the first fluid conduit 16 and the cylinder 32.
A fluid control module, generally designated F, receives high pressure refrigeration fluid from the compression unit 12 through the port 44. Elements of the cylinder housing 30 and the gas displacing piston 36 combine to provide a clearance seal at the open end of the cylinder 32, which prevents refrigeration gas from escaping from the cylinder 32 while still allowing movement of the gas displacing piston 36. The gas displacing piston 36 is configured with internal fluid passages 46 extending from the port 44 to a regenerator R, described below.
A regenerator module R comprises an insulating regenerator tube 48 formed as a fluid conduit and filled with a regenerator matrix 50 comprising a porous solid material configured to exchange thermal energy with the refrigeration gas as the gas flows through the regenerator tube 48. The regenerator module R receives incoming warm refrigeration gas at high pressure from the fluid control module F. The refrigeration gas flows through the regenerator tube 48 and exchanges thermal energy with the regenerator matrix 50 before flowing into the expansion space 40. On a return path, cold low pressure refrigeration gas exiting from the expansion space 40 flows through the regenerator module R, cooling the regenerator matrix 50 before flowing back to the compression unit 12.
A thermal barrier T, designated schematically by the dashed line in FIG. 1, comprises one or more thermally insulating elements disposed to prevent thermal conduction across the thermal barrier T. Generally elements on the warm side of the thermal barrier T are at the local ambient temperature, or a higher temperature due to heat dissipation in the compression unit 12 and drive motors, not shown, and elements on the cold side of the thermal barrier T are below the ambient temperature. During operation, the expansion space 40, also called a cold tip or cold end, is maintained at a cryogenic temperature, e.g. 77° K., while the fluid control module F and the compression unit 12 remain substantially at the local ambient temperature, e.g. 270° K. Accordingly, a very steep thermal gradient extends along the longitudinal length of the regenerator module R.
It is well understood that using a regenerator module R to pre-cool refrigeration gas or another working fluid as it flows from the compression unit 12 to the expansion space 40 increases the cooling power that can be delivered to the element to be cooled 42. In addition, pre-heating refrigeration gas as it flows from the expansion space to the compressor improves the efficiency of the refrigeration device. Ideally a regenerator module R is designed for 100% effectiveness which means that the regenerator module completely pre-cools, or pre-heats, the refrigeration gas flowing along its length. In particular, 100% effectiveness occurs when warm refrigeration gas entering the regenerator module at the warm end exits the regenerator module at the cold end at the cooling temperature of the device, e.g. 77° K. When this is the case, substantially all of the cooling power generated by expanding the expansion space 40 volume is available to be delivered to the device to be cooled 42 and none of the cooling power generated by the device is needed to further cool the entering refrigeration gas. Conversely, 100% effectiveness occurs when cold refrigeration gas entering the regenerator module at the cold end exits the regenerator module at the warm end at the local ambient temperature, e.g. 270° K. When this is the case, substantially all of the cooling available from the cold refrigeration gas is transferred to the regenerator matrix 50. Analytical models have shown that any reduction in the effectiveness of the regenerator greatly degrades the cooling power of the refrigeration device. In one example, Applicants calculated that a conventional refrigeration device of the type shown in FIG. 1 may be reduced to 80% of its potential cooling power when the regenerator matrix is 99% effective instead of 100% effective.
It is further understood that the effectiveness of a regenerator is a function of the magnitude of the total surface area of surfaces of the regenerator matrix substrate that contact working fluid and further that the total surface area is strongly dependent upon the longitudinal length L of the regenerator module R. Heretofore is has been a hard design requirement of a miniature cryocooler refrigeration system that the regenerator matrix 50 be configured with sufficient longitudinal length L for making a 100% effective thermal energy exchange with the refrigeration gas flowing along its length. However this hard design requirement is in conflict with reducing the size of the refrigeration device 10.
Generally there is a need in the art to further miniaturize refrigeration devices or at least to further miniaturize the volume control unit 14 to deliver cooling power to smaller elements to be cooled 42 or to fit the refrigeration device 10 or the volume control unit 14 within smaller volume enclosures. A major barrier to reducing the size of the refrigeration device 10 or the size of the volume control unit 14 has been an inability to reduce the longitudinal length L of the regenerator matrix 50 while still providing a 100% thermal energy exchange with the working fluid.
Heretofore, miniature refrigeration devices like the one shown in FIG. 1 have employed a single regenerator matrix 50 disposed in the regenerator module R and more specifically with the entire longitudinal length L of the regenerator module disposed on the cold side of the thermal barrier T. Such a system configuration is not easily miniaturized. According to the present invention, the overall size of a refrigeration device is reduced by configuring the device with a longitudinal length L of a regenerator matrix disposed on the cold side of the thermal barrier to a length L that is less than a length L required for 100% effectiveness and other regenerator modules are disposed on the warm side of the thermal barrier T to add further regenerating capacity as may be required to provide 100% regenerator effectiveness.